Slownz Slownz - 3 months ago 8
R Question

R regular expression: find the last but one match

I need a regular expression script (in R language) which finds the last but one match.

Here is an example:

input = c("(test1(test2(test3","(((((othertest1(othertest2(othertest3")
regexpr('the_right_regular_expression_here_which_can_finds_the_last_but_one_'(' ', input)


The result has to be: 7 and 16, because in the first case the last but one '(' is in a 7th position (from left), and in the second case the last but one '(' in in the 16th position (from left).

I've found a regular expression which can find the last match, but I could not transform it in the right way:

\\([^\\(]*$


Thanks for any help!

Answer

To match a chunk of text beginning with the last but one (, you may use

"(\\([^(]*){2}$"

Details:

  • (\\([^(]*){2} - 2 sequences of:
    • \( - a literal (
    • [^(]* - zero or more chars other than (
  • $ - end of string.

R test:

> input = c("(test1(test2(test3","(((((othertest1(othertest2(othertest3")
> regexpr("(\\([^(]*){2}$", input)
[1]  7 16
attr(,"match.length")
[1] 12 22
attr(,"useBytes")
[1] TRUE
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